Reduction And Removal Of Cr(â…¥) In Soil By NZVI Stabilized By Galacturonic Acid-CMC

Soil pollution prevention are the key work of environmental protection of China in the recent stage. Soil and site pollutants are involved in a wide range and difficult to control. They are generally concealed and have hysteresis quality. The research on soil pollution and contaminated soil remediation has very important practical significance and the long-term value. The heavy metal pollution is one of the core issues of the soil pollution prevention.Although there are lots of soil remediation technologies in recent years, including the physical, chemical and biological treatment methods, etc, there are rarely technologies costing little but working efficiently. The nanoscale zero-valent iron(n ZVI) in-situ soil remediation technology belongs to chemical reduction methods, with lots of advantages such as operating simply, costing little, having extremely broad scope, repairing quickly and working efficiently, etc. On the one hand, in recent years,there are lots of people researching on removing Cr(VI) with n ZVI in recent years, but most of them are limited to reduce Cr(VI) in water, or research on sandy soil. They seldom research on transmission and reaction performance of Fe~0 in practical soil and application parameter of this technology in loam, loess and red clay in the north of China.So they even more lack researching on application example of chrome residue contaminated sites ‘ s on-site soil. On the other hand, although there have been some research about the adoption of CMC as stabilizers of Fe~0, but it lack the actual application example of CMC-Fe~0 in the practical soil. And CMC is limited in acidity and basicity and don‘t have precise dosages as stabilizers. General nanometer iron powder is so active that it always gather into cluster. It‘s vital to choose suitable stabilizers, dispersants, surfactants, effective dosing methods and dosing concentration to stabilize nanometer iron in promotion of n ZVI.In this paper, the main research contents include:Use "GAM+CMC" complex solution as the complex stabilizer of n ZVI, and research on the effect and mechanism of adopting stabilizing n ZVI to reduce and immobilize Cr(VI) in the soil. The research on basic properties of stabilizing n ZVI ?s reaction in water includes the p H value of stabilizer solution, the dispersion of the nanometer iron solution, the effect of removing Cr(VI) in water with all kinds of stabilizer systems, and the influencing factors of removing Cr(VI) the liquid phase with stabilizing n ZVI. Adopting soil static batch experiments to research on the basic problems of removing Cr(VI) in the static soil sample.It mainly includes: the effect of removing Cr(Ⅵ) in the soil static sample with different n ZVI systems, the influence of initial injecting concentration, dosing concentration of stabilizing n ZVI, initial p H value, temperature of system, organic content in the experiments of removing Cr(VI) in static soil, and the static experiment of the effect that stabilizing n ZVI remove Cr(VI) in different soil sample. Use a variety of soil column experiment to simulate the operating conditions in actual soil sites, and the transmission characteristics of stabilizing n ZVI in soil medium, the elution process rule of Cr(VI) in the soil column, long-time maintained experiment rule of soil column and the occurrence rule of Cr. The basic experimental method in this paper Including : the preparation of stabilization n ZVI by liquid phase chemical reduction, spectrophotometric Determination of Cr(VI),detection of turbid solution morphology of environmental scanning electron microscope(ESEM),transmission electron microscopy(TEM) detection of suspended particle shape, the dispersion of ultraviolet visible spectrophotometer measuring suspension system, study on Cr(VI) treatment effect and control parameters through static soil sample test, study on the mechanism of the soil with stabilization n ZVI through dynamic simulated experiment of soil column, determination of various chemical forms of Cr in soil through soil sample digestion step by step combined with atomic absorption spectrophotometry.The main conclusions of this paper include:(1) A mixture of galacturonic acid- CMC solution is advantageous to the reduction of Cr(VI) with n ZVI and suitable for decentralized stabilization of n ZVI. With stable n ZVI suspension system of complex stabilizer galacturonic acid and CMC, it‘s advantageous to the effect of the reduction of Cr(VI). Within 5 min when the reaction started, Cr(VI) ?s removal efficiency is up to 83.1%, and within 60 min removal efficiency can reach 94.7%.(2) GAM+CMC complex stabilizer can improve the effective threshold stabilizer adds. Galacturonic acid can promote CMC ’s scatter, can offset part of the thickening effect of CMC. And it’s advised to set the proportion of the stabilizer and n ZVI(mass ratio) as 10:1. When the dosage of stabilizing n ZVI is between 0.02g/L and 0.10 g/L, lager the dosage is, higher the final removal rate of Cr(VI) gets, and the proposal for the dosage of galacturonic acid- CMC stabilizing n ZVI is 0.06g/L. When using GAM+CMC system as n ZVI ’s dispersion stabilizer, initial tolerated p H value of the alkaline gets higher than using simple CMC. When the p H value is too low(< 3.5), reducing efficiency of Cr(VI) drops sharply. The inhibiting effect of strong acid on the system of this experiment is greater than strong alkaline condition, and strong alkaline condition is only the restricting reason. The proposal for initial p H value is 6.0.The anti-load ability of n ZVI when it meets with Cr(VI) was measured based on the Cr/Fe load rate. The load rate has great effect on the removal rate of Cr(VI). In the same kind of system, the maximum of the load rate suggested is 0.10 mg/mg.(3) The experiment of using n ZVI reducing Cr(VI) in the static soil samples showed that different n ZVI reaction system would reach a stable state in 18 hours. The concentration of Cr(VI) in the transudates of GAM+CMC stabilizing n ZVI static reaction system in 6 hours is 0.55 mg/L, which is far lower than the other reaction system.The initial content of stabilizing n ZVI has little effect on the static batch experiment in the soil and the removal efficiency of Cr(VI) in the soil is a little lower than in the water. The different concentration of injection can lead to distinct removal rate rules. The removal rate can be higher if the injection concentration of stabilizing n ZVI is 0.08 g/L. During soil static batch experiments, galacturonic acid can promote the stability of CMC compounding reaction more obviously. The suggested dosage of GAM+CMC stabilizer is 1.0 g/L, in other words, the proportion of the compound stabilizer to n ZVI is 10:1.(4) In the static soil sample experiment, higher the initial p H value is, lower the conversion and attachment rate of Cr(VI) gets, but the batch test results in the soil sample are more stable than in the water. When the p H value is too low(< 3.5), the reduction efficiency of Cr(VI) declined slightly, the batch test results in the soil sample declined less than in the water, and the basic range of the tolerant initial PH value in the soil samples is wider than in the water. In the experiment of the static soil sample, the increase of reaction temperature will lead to the increase of the reaction rate, and the removing effect of Cr(VI) gets better. Low temperature have inhibitory effect on the reaction, and the suggested temperature of the reaction system is 25 ℃. The more organic content in soil, the less Cr(VI) are removed.The rules of performance in the static experiments of using stabilization n ZVI to reduce and immobilize Cr(VI) in various soil are obviously distinct. In the static soil experiment, arrange the conversion and attachment rate of Cr(VI) from large to small: sandy soil > chrome slag field soil > half sandy loam > calcification red clay > loess > loam.(5) The results of the soil column experiment showed that the sandy soil and half sandy loam, chrome slag field soil could be penetrated very easily, calcification red clay could be penetrated easily, the penetration rate of loess and loam is slower. The elution process rules of Cr(VI) in different soil sample columns shows, arrange the elution continuity time from short to long: sandy soil > chrome slag field soil > half sandy loam > calcification red clay > loess > loam. The order of hysteresis and degeneration reaction degree that the soil effect on Fe~0 is: loam > chrome slag field soil > calcification red clay > half sandy loam > loess > sandy soil.(6) After lasting a long time in the practical soil, the n ZVI reduction effect of Cr(VI) will fluctuate in a certain range, and the removal rate of Cr(VI) will get lower. The uronic acid- CMC complex stabilizer can promote the overall effect of n ZVI reducing Cr(VI). When the dosing concentration of n ZVI is 0.37g/L, the eventual conversion and attachment rate of Cr(VI) is 97.55% in the chrome slag field soil. It makes sense to use excess n ZVI to sustain the reducing effect of reaction system.(7) The occurrence of Cr during n ZVI reaction process shows that the interference factors in practical soil samples is too many to the adsorption states. The occurrence distributions of Cr in different soil are distinct, but on the whole, the residual state accounts for the largest proportion, followed by which are the adsorption state and the organic combination state. As for the rules of the Cr state changing in the soil, deeper the residual state is, lager its proportion gets. Deeper the carbonate bounded state is, lager its proportion of mid-depth gets. Deeper the organic bounded state, smaller its proportion gets. The valence state of Cr, the proportion of its various valence state, reaction time, p H value, the content of organic matter in the soil, the preferential flow in the soil, and other hydraulic conditions could cause the change of the occurrence of Cr, and in turn these changes of the occurrence restrict the conversion and attachment rate of Cr(VI).This paper carried on the beneficial exploration to deepen the understanding of n ZVI reducing and remedying mechanism of Cr(VI), and has a certain reference value to master the stabilization technology of n ZVI, and preliminary make clear the application parameter and mechanism of n ZVI reducing Cr(VI) in the loam, loess, calcification red clay and chrome slag field soil.